Murder in Baldur's Gate
Coren suggests the party investigates an old manor that appears to be haunted, as that would be an excellent place to find cultists.
After clearing the mansion of undead the party found that it was not being occupied by cultists. Instead it was a deranged wizard who turned himself into a spectre while trying to raise his dead wife. There was evidence that at some point The Guild was supplying him via tunnels, as a letter from The Fetcher was found near some old rotting supplies by a collapsed tunnel.